1. Field of the Invention
The present invention relates to an outboard motor, and more particularly to an outboard motor having a transmission ratio changing unit for changing the output from an engine that is transmitted to a propeller.
2. Description of the Related Art
One example conventional outboard motor of this type is described in Japanese Patent No. 2686517,which discloses an outboard motor having a first drive shaft connected to the output shaft of the engine, a second drive shaft disposed coaxially with the first drive shaft via a transmission ratio changing unit, and a speed reducing and reversing mechanism for transmitting torque transmitted to the second drive shaft to a propeller shaft.
The transmission ratio changing unit comprises first and second planetary gear trains each including a sun gear, a plurality of planetary gears, and an internal gear with which the planetary gears are in meshing engagement.
The axes of rotation of the sun gears of the first and second planetary gear trains are connected by a rotating spindle to integrate them, and a brake mechanism which is operable from outside is disposed adjacent to the rotating spindle. The first drive shaft transmits its torque to the internal gear of the first planetary gear train, and the rotation of an arm for the planetary gears of the first planetary gear train, which is rotated by the torque, is transmitted to the second drive shaft. The arm for the planetary gears of the first planetary gear train and the internal gear of the second planetary gear train are integrated with each other, and a one-way clutch which allows an arm for the planetary gears of the second planetary gear train to rotate in a predetermined direction is provided.
In the above configuration, when the brake mechanism is activated, the speed reduction ratio of the torque to be transmitted to the second drive shaft becomes smaller than before activation of the brake mechanism.
Some outboard motors mounted on boats have a first propeller and a second propeller arranged in tandem in opposed relationship and a contra-rotating propeller mechanism for rotating the first propeller and the second propeller in directions opposite to each other as disclosed in, for example, Japanese Patent Documents JP-A-Hei 7-144695 and JP-A-Hei 9-263294.
Although exhaust gas is discharged into water in both Patent Document '695 and Patent Document '294,exhaust gas is discharged from inside of a propeller cylinder in Patent Document '695 and from outside of a propeller cylinder in Patent Document '294, during acceleration.
Such conventional outboard motors, however, have complicated structures, and a large installation space is required since the transmission ratio changing unit has first and second planetary gear trains with one arranged above the other.
One feature of such a contra-rotating propeller system is that it has very excellent propeller cavitation performance since it has two propellers for producing thrust and thus the total area of the propeller vanes is greater than that of a single propeller system having one propeller for producing thrust. Because of this feature, free spinning of the propellers seldom occurs during acceleration or deceleration of the boat. Therefore, the outboard motor is excellent in starting and acceleration performance and deceleration performance.
Also, one characteristic of a contra-rotating propeller engine is that the required torque, especially on the low-speed side, typically is very large as compared to a single propeller engine having one propeller for producing thrust all the more because free spinning of the propellers seldom occurs. Therefore, a means for increasing the engine output such as a turbo charger or super charger typically is added. However, since there is a limit to the amount by which the torque can be increased and since there is naturally a response time lag between an operation to increase the output and an actual increase in the engine output, the feature of a contra-rotating propeller system cannot be fully exploited. In addition, such a means tends to lower the traveling mileage.
In Japanese Patent Document JP-A-Hei 9-263294,a structure in which exhaust gas discharged from a propeller boss part swirls around the propeller vanes is employed in order to reduce the required torque caused by free spinning of the propellers. This method, however, produces a result in which the high water catching ability of the contra-rotating propeller system is not fully exploited.
In general, slowly rotating a propeller with a relatively large diameter and a high pitch is preferred in terms of the propeller efficiency and results in improvement of the maximum speed and traveling mileage. However, a propeller with a higher pitch requires larger propeller driving torque at the time of acceleration, and great improvement of the traveling performance by increasing the propeller pitch cannot be expected when a low-speed torque improving means such as a turbo charger as mentioned before is employed.